1. Field
Example embodiments relate to a semiconductor memory device and a method of fabricating the same. Other example embodiments relate to a semiconductor memory device and a method of fabricating the same, in which the contact resistance of the device may be reduced.
2. Description of the Related Art
The design rule of a semiconductor memory device may be reduced as a higher-level integration of the semiconductor memory device is needed. The size of a contact for connecting lines in the semiconductor memory device may be reduced and this may cause an aspect ratio of the contact to increase. A contact may be made of tungsten (W) that has an improved step coverage and relatively low resistance. When a contact hole is filled with the tungsten as a contact-forming material, an ohmic layer may be formed to relieve a difference in electrical resistance between the tungsten and an interlayer dielectric film, for example, a silicon oxide film.
A barrier layer may then be formed on the ohmic layer so as to reduce or prevent damage to the ohmic layer. A titanium silicide (TiSiX) film and a titanium nitride (TiN) film may be used as the ohmic layer and the barrier layer, which may be formed before the contact hole is filled with the tungsten. As an aspect ratio of the contact increases, the thickness of the titanium silicide film and the titanium nitride film may also be increased and this may cause the contact resistance to be increased. An ohmic tungsten (W) film and a tungsten nitride (WN) film may be used instead of the titanium silicide film and the titanium nitride film. The ohmic tungsten film and the tungsten nitride film may be formed using B2H6 and WF6 gases.
If a polysilicon or silicon germanium substrate doped with n-type impurities is used as a lower substrate when the tungsten film and the tungsten nitride film are formed using the B2H6 gas, the substrate may be counter-doped with boron ions. Accordingly, the contact resistance of the semiconductor memory device may be increased and this may cause the contact to be unstable.